A new approach to integration of mineralogy, geochemistry and texture for improving prediction of acid metalliferous drainage from abandoned mines

Current predictive methodologies used to classify the acid forming potential of mine wastes are dominated by laboratory-based geochemical tests and mineralogical evaluations. Textural analyses are largely absent from these protocols despite the direct control of texture on acid formation. This study introduces the use of a three-stage geochemical, mineralogical and textural (GMT) approach where results are evaluated concurrently to classify acid forming potential. A simple textural evaluation scheme- the ARD Index (ARDI) was developed as part of this approach. The ARDI evaluated a range of intact samples in terms of their acid forming or neutralising potential based on five key parameters (A- sulphide content, B- degree of sulphide alteration, C- sulphide morphology, D- content of neutralising minerals and E- spatial association of neutralising minerals and sulphides).

The GMT approach and ARDI were tested on waste rock samples obtained from the abandoned Croydon Au-mining operations, North Queensland. Acid rock drainage is emanating from these operations causing elevated downstream concentrations of cadmium and zinc relative to the local baseline. Ten mesotextural groups (A-J) were identified from the sampled waste rock. GMT assessments confirmed three groups as environmentally significant in terms of acid metalliferious drainage. Group J (quartz-pyrite) is the most acid generating with the greatest heavy metal contents. Group G (quartz-galena-sphalerite) was identified as the main source of cadmium. Group H (arsenopyrite-pyrite-quartz) contains the greatest quantity of metalloids and is also extremely acid forming. These groups have been selected for detailed kinetic test work.

At the end of stage-one, sample classifications were in agreement with those assigned at the end of stages-two and –three, indicating that for abandoned mine sites such as Croydon, classification of waste rock in terms of acid forming potential could be made based on lower-cost stage-one tests alone.